1. Field of the Invention
This invention relates generally to the use of ultrasound techniques for the measurement of sound velocity in organisms. More particularly, the invention relates to an apparatus and method for the estimation of sound velocity in medical diagnostics. The invention is especially concerned with measuring the travel time of ultrasonic pulses across known distances in the human body.
2. Background Art
X-ray diagnostic techniques, because of their increased toxicity to organic tissue, have been supplanted in many applications by ultrasound. Ultrasound has proved to be highly effective for producing images of the internal organs of the body. In addition, it has been found that certain pathological conditions in living organs cause changes in the velocity at which sound propagates in the organ. For example, cirrotic liver tissue is more dense than normal liver tissue. The velocity of sound in cirrotic tissue is therefore greater than that in normal tissue. Similarly, the increased tissue density in the region of tumors will result in an increased sound velocity in the tumor region. Measurement of sound velocity in the organ can thus be useful in the diagnosis of disease.
A simple method of sound velocity measurement involves the transmission of sound pulses through tissue regions of known dimension and recording of the time required for the pulse to traverse the region. The quotient of travel distance and travel time is computed to yield the velocity.
The transmission and reception of sound energy in medical diagnostics is accomplished with ultrasound transducers. These transducers contain piezoelectric crystals which produce an electric potential difference proportional to the amount by which they are physically deformed. Conversely, the application of an electric potential difference across the crystal causes a proportional physical deformation. As a consequence, the transducer may be used as both a transmitter and a receiver of ultrasound energy. An example of a practical embodiment of ultrasound transducers used in the medical field is described in an aritcle entitled "The U.I. Octoson--A New Class of Ultrasonic Echoscope" by Carpenter, et al., in Australasian Radiology, Vol. 21, No. 1 (1977) pp. 85-89.
These transducers are characterized by high directivity. In the transmit mode, a transducer will produce sound in a relatively narrow beam surrounding the transducer's axis of radiation. The transducer is correspondingly directional in the receive mode in that it will only respond to sound energy directed to it along a line nearly coincident to its axis of radiation.
One configuration that has been used in medical diagnostics requires a transmitting transducer and a separate receiving transducer arranged so that they are aimed at one another with their respective axes of radiation coincident. The body of the subject is placed between the transmitting and receiving transducers. Acoustical coupling may be accomplished in one of two ways. The subject may be required to stand immersed in a tank of water so that the water effects a coupling to the transducers. Alternatively, the transducers may be attached directly to the skin of the subject on either side of the body or an appendage thereof, and an acoustical coupling agent applied between the transducer and the skin to insure tight acoustical coupling.
The two-transducer configuration may be effective when used with body appendages, such as the breast or testes, where the transducers may be attached directly on either side of the organ of interest. Often, organs such as the liver are not so readily accessible. Use of the above recited configuration will require placement of transducers on opposite sides of the body and will require the ultrasound energy to traverse other organs and bone as well as the organ of interest. The quotient of distance and travel will thus yield a weighted average of the velocities in the various organs and bone traversed. In addition, refraction will occur at each tissue interface resulting in a bending of the path of the incident ultrasound pulse off the axis of radiation of the receiving transducer.
A second configuration that has been used in medical diagnostics is a modification of the first-described configuration in which a single transducer serves as both transmitter and receiver. A sound mirror, such as a steel plate, is placed in the position occupied by the receiving transducer in the first-described configuration and the transducer formerly used only for transmitting is now used also as a receiving transducer.
This second configuration does have a slight advantage over the first configuration in that only one transducer is required. However, the previously-described disadvantages still remain. Moreover, the sound pulse is now required to traverse the entire cross-section of the body twice, once as an incident wave, and a second time as a reflected wave. Consequently, attenuation of the pulse is doubled. The previously-recited problem with acoustical refraction is also exacerbated in that the pulse path must now undergo bending not only for the path of the incident wave, but for the path of the reflected wave as well.
A more sophisticated technique for sound velocity estimation is disclosed by Robinson, Chen and Wilson in "Image Matching for Pulse Echo Measurement of Ultrasound Velocity", published in Image and Vision Computing, Volume 1, Number 3, Aug., 1983. In this method, sound images of a tissue region are obtained from different angles. Sound velocity may be determined from the difference in position of the same feature in different images. This method works best when a well defined feature is available. In simulated tissue regions, known as "phantoms", a thin wire added to the region will provide such a well defined feature. Features normally appearing in living tissue are not as well defined and the resulting measurement is therefore not as accurate.
Accuracy in sound velocity estimation is extremely important in the analysis of tissue for pathological conditions. Using the liver as an example, sound velocity will be fairly uniform with variations due to both natural inhomogeneities and pathological conditions amounting to about 5%. Accuracy of estimation of velocity must be at least 0.5% provide adequate diagnosis of organ disease. Many factors affect the measurement accuracy, including the refraction occuring at the tissue interfaces, the limited accessibility of some organs, and the dispersive attenuation of ultrasound energy by the tissue itself. Sound velocity estimation in organic tissue requires a technique capable of overcoming these limitations while being relatively simple to implement and apply.